I would like to use Valentina Reports to print a ticket in an Epson TM20-II Thermal Receipt Printer. This printer uses 80mm Thermal receipts paper rolls. Would it be possible to create a report or ticket with my product list in Valentina Reports?. The main problem is the 'Paper format' because I would need a specific width (80 mm) but the height would be variable (depending on the size of the product list). Would it be possible to get this custom page with Valentina Reports?

Yes, it's possible.
1) Create a custom paper format with a specific width, any height (it will be ignored) and set it for the report.
2) Check the report property ignore_pagination in the Property Inspector.